Principles of Heat Treatment

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Guidelines for Shop Inspection Principles of Heat Treatment

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PRINCIPLES OF HEAT TREATMENT

PRINCIPLES OF HEAT TREATMENT of METALS........................................................ 2

Heating .................................................................................................................................. 2

Soaking .................................................................................................................................. 3

Cooling................................................................................................................................... 3

FORMS OF HEAT TREATMENT........................................................................................ 4

Annealing .............................................................................................................................. 4

Normalizing........................................................................................................................... 5

Hardening ............................................................................................................................. 5

Tempering ............................................................................................................................. 5

Case Hardening .................................................................................................................... 5

HEAT TREATMENT OF FERROUS METALS (STEEL)................................................. 7

Principles of Heat Treatment of Steel ................................................................................ 7

Forms of Heat Treatment of Steel ...................................................................................... 7

Hardening ......................................................................................................................... 7

Tempering ....................................................................................................................... 10

Annealing and Normalizing........................................................................................... 11

Case Hardening .............................................................................................................. 12

HEAT TREATMENT OF NONFERROUS METALS (ALUMINUM ALLOYS).......... 14

Heat Treating Procedures ................................................................................................. 14

Solution heat treatment ................................................................................................. 15

Precipitation hardening (aging hardening).................................................................. 15

Quenching ....................................................................................................................... 15

Annealing ........................................................................................................................ 16


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PRINCIPLES OF HEAT TREATMENT of METALS

Heat treatment is a series of operations involving the heating and cooling of a metal or alloy in the solid state

for the purpose of obtaining certain desirable characteristics. The rate of heating and cooling determines

the crystalline structure of the material. In general, both ferrous metals (metals with iron bases) and nonferrous

metals, as well as their alloys, respond to some form of heat treatment. Almost all metals have a critical

temperature at which the grain structure changes. Successful heat treatment, therefore, depends largely on

a knowledge of these temperatures as well as the time required to produce the desired change

The results that may be obtained by heat treatment depend, to a great extent, on the structure of the metal and the

manner in which the structure changes when the metal is heated and coded. A pure metal cannot be hardened by

heat treatment because there is little change in its structure when heated. On the other hand, most alloys respond

to heat treatment because their structures change with heating and cooling.

An alloy may be in the form of a solid solution, mechanical mixture, or a combination of a solid solution and a

mechanical mixture. When an alloy is in the form of a solid solution, the elements and compounds that form the

alloy are absorbed, one into the other, in much the same way that salt is dissolved in a glass of water. The

constituents cannot be identified even under a microscope.

When two or more elements or compounds are mixed, but can be identified by microscopic examination, a

mechanical mixture is formed. A mechanical mixture might be compared to the mixture of sand and gravel in

concrete. The sand and gravel are both visible. Just as the sand and gravel are held together and kept in place by

the mixture of cement, the other constituents of an alloy are embedded in the mixture formed by the base metal.

An alloy that is in the form of a mechanical mixture at ordinary temperatures may change to a solid solution

when heated. When cooled back to normal temperature, the alloy may return to its original structure. On theother hand, it may remain a solid solution or form a combination of a solid solution and mechanical mixture. An

alloy that consists of a combination of a solid solution and mechanical mixture at normal temperatures may

change to a solid solution when heated. When cooled, the alloy may remain a solid solution, return to its original

structure, or form a complex solution.

Heat treatment involves a cycle of events. These events are heating, generally done slowly to ensure uniformity;

soaking, or holding the metal at a given temperature for a specified length of time; and cooling, or returning themetal to room temperature, sometimes rapidly, sometimes slowly. These events are discussed in the following

paragraphs.

Heating

Uniform temperature is of primary importance in the heating cycle. If one section of a part is heated more

rapidly than another, the resulting uneven expansion often causes distortion or cracking of the part. Uniform

heating is most nearly obtained by slow heating. The rate at which a part maybe heated depends on several

factors. One important factor is the heat conductivity of the metal. A metal that conducts heat readily may be

heated at a faster rate than one in which heat is not absorbed throughout the part as rapidly. The condition of the

metal also affects the rate at which it may be heated. For example, the heating rate for hardened tools and parts

should be slower than for metals that are not in a stressed condition. Finally, size and cross section have an

important influence on the rate of heating. Parts large in cross section require a slower heating rate than thin

sections. This slower heating rate is necessary so that the interior will be heated to the same temperature as the

surface. It is difficult to uniformly heat parts that are uneven in cross section, even though the heating rate is

slow. However, such parts are less apt to be cracked or excessively warped when the heating rate is slow.


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Soaking

The object of heat treating is to bring about changes in the properties of metal. To accomplish this, the metal

must be heated to the temperature at which structural changes take place within the metal. These changes occur

when the constituents of the metal go into the solution. Once the metal is heated to the proper temperature,

it must be held at that temperature until the metal is heated throughout and the changes have time to take place.

This holding of the metal at the proper temperature is called SOAKING. The length of time at that

temperature is called the SOAKING PERIOD. The soaking period depends on the chemical analysis of themetal and the mass of the part. When steel parts are uneven in cross section, the soaking period is

determined by the heaviest section. In heating steels, the metal is seldom raised from room temperature to

the final temperature in one operation. Instead, the steel is slowly heated to a temperature below the

point at which the solid solution begins, and it is then held at that temperature until heat is absorbed

throughout the metal. This process is called PREHEATING. Following the preheating, the steel is quicklyheated to the final temperature. Preheating aids in obtaining uniform temperature throughout the part being

heated, and, in this way, reduces distortion and cracking. When apart is of intricate design, it may have to be

preheated at more than one temperature to prevent cracking and excessive warping. As an example, assume that

an intricate part is to be heated to 1,500F (815C) for hardening. This part might be slowly heated to 600F

(315C), be soaked at this temperature, then be heated slowly to 1,200F (649C), and then be soaked at that

temperature. Following the second preheat, the part would be heated quickly to the hardening temperature.

Nonferrous metals are seldom preheated because they usually do not require it. Furthermore, preheating tends to

increase the grain size in these metals.

Car Bottom Ingot Soaking for Billet Heating

Cooling

After being heated to the proper temperature, the metal must be returned to room temperature to complete the

heat-treating process. The metal is cooled by placing it in direct contact with a gas, liquid, or solid, or somecombination of these. The solid, liquid, or gas used to cool the metal is called a "cooling medium." The rate at

which the metal should be cooled depends on both the metal and the properties desired. The rate of cooling also


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depends on the medium; therefore, the choice of a cooling medium has an important influence on the properties

obtained.

Cooling metals rapidly is called quenching and the oil, water, brine, or other mediums used for rapid

cooling is called a quenching medium. Since most metals must be cooled rapidly during the hardening process,

quenching is generally associated with hardening. However, quenching does not always result in an

increase in hardness. For example, copper is usually quenched in water during annealing. Other metals, air-

hardened steels for example, may be cooled at a relatively slow rate for hardening.

Some metals are easily cracked or warped during quenching. Other metals may be cooled at a rapid rate with no

ill effects. Therefore, the quenching medium must be chosen to fit the metal. Brine and water cool metals

quickly, and should be used only for metals that require a rapid rate of cooling. Oil cools at a slower rate and is

more suitable for metals that are easily damaged by rapid cooling. Generally, carbon steels are considered waterhardened and alloy steels oil hardened. Nonferrous metals are usually quenched in water.

FORMS OF HEAT TREATMENT

The various heat-treating processes are similar in that they involve the heating and cooling of metals. They

differ, however, in the temperatures to which the metals are heated, the rates at which they are cooled, and, of

course, in the final result. The most common forms of heat treatment for ferrous metals are annealing,

normalizing, hardening, tempering, and case hardening. Most nonferrous metals can be annealed but never

tempered, normalized, or case hardened. Successful heat-treating requires close control over all factors affecting

the heating and cooling of metals. Such control is possible only when the proper equipment is available, and the

equipment is selected to fit the particular job.

Annealing

Annealing is used to reduce residual stresses, induce softness, alter ductility, or refine the grain structure.

Maximum softness in metal is accomplished by heating it to a point above the critical temperature, holding at

this temperature until the grain structure has been refined, followed by slow cooling.

Coil Annealing Furnace using a hot car design for coilhandling
http://www.secowarwick.com/aluminumprocess.html


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Normalizing

Normalizing is a process whereby iron base alloys are heated to approximately 100?F (56?C) above the upper

critical temperature, followed by cooling to room temperature in still air. Normalizing is used to establish

materials of the same nature with respect to grain size, composition, structure, and stress.

Single Zone Annealing Furnaces

Hardening

Hardening is accomplished by heating the metal slightly in excess of the critical temperature, and then rapidly

cooling by quenching in oil, water, or brine. This treatment produces a fine grain structure, extreme hardness,

maximum tensile strength, and minimum ductility. Generally, material in this condition is too brittle for most

practical uses, although this treatment is the first step in the production of high-strength steel.

Tempering

Tempering (drawing) is a process generally applied to steel to relieve the strains induced during the hardening

process. It consists of heating the hardened steel to a temperature below the critical range, holding this

temperature for a sufficient period, and then cooling in water, oil, or air. In this process, the degrees of strength

hardness and ductility obtained depend directly upon the temperature to which the steel is heated. Hightempering temperatures improve ductility at the sacrifice of tensile, yield strength, and hardness.

Case Hardening

The objective in casehardening is to produce a hard case over a tough core. Casehardening is ideal for parts that

require a wear-resistant surface and, at the same time, must be tough enough internally to withstand the applied

loads. The steels best suited to case hardening are the low-carbon and low-alloy steels. If high-carbon steel iscase-hardened, the hardness penetrates the core and causes brittleness. In case hardening, the surface of the metal

is changed chemically by inducing a high carbide or nitride content. The core is unaffected chemically. When

heat treated, the surface responds to hardening while the core toughens. The common methods of case hardening

are carburizing, nitriding, and cyaniding.


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CarburizingCarburizing consists of holding the metal at an elevated temperature while it is in contact with a solid or

gaseous material rich in carbon. The process requires several hours, as time must be allowed for the surface

metal to absorb enough carbon to become high-carbon steel. The material is then quenched and tempered to

the desired hardness.

NitridingNitriding consists of holding special alloy steel, at temperatures below the critical point, in anhydrous ammonia.

Absorption of nitrogen as iron nitride into the surface of the steel produces a greater hardness than carburizing,

but the hardened area extends to a lesser depth.

Cyaniding

Cyaniding is a rapid method of producing surface hardness on an iron base alloy of low-carbon content. Itmay be accomplished by immersion of the steel in a molten bath of cyanide salt, or by applying powdered

cyanide to the surface of the heated steel. The temperature of the steel during this process should range from

760 to 899C (1,400 to 1,650F), depending upon the type of steel, depth of case desired, type of cyanide

compound, and time exposed to the cyanide. The material is dumped directly from the cyanide pot into the

quenching bath.


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HEAT TREATMENT OF FERROUS METALS (STEEL)

The first important consideration in the heat treatment of a steel part is to know its chemical

composition. This, in turn, determines its upper critical point. When the upper critical point is known, the

next consideration is the rate of heating and cooling to be used. Uniform-heating furnaces, proper temperature

controls, and suit able quenching mediums are used in carrying out these operations

Principles of Heat Treatment of Steel

Changing the internal structure of a ferrous metal is accomplished by heating it to a temperature above its upper

critical point, holding it at that temperature for a time sufficient to permit certain internal changes to occur, and

then cooling to atmospheric temperature under predetermined, controlled conditions. At ordinary temperatures,the carbon in steel exists in the form of particles of iron carbide scattered throughout the iron mixture known as

ferrite. The number, size, and distribution of these particles determine the hardness of the steel. At elevated

temperatures, the carbon is dissolved in the mixture in the form of a solid solution called "austenite," and the

carbide particles appear only after the steel has been cooled. If the cooling is slow, the carbide particles are

relatively coarse and few. In this condition the steel is soft. If cooling is rapid, as by quenching in oil or water,

the carbon precipitates as a cloud of very fine carbide particles, and the steel is hardened. The fact that the

carbide particles can be dissolved in austenite is the basis of the heat treatment of steel. The temperatures at

which this transformation takes place are called the "critical points," and vary with the composition of the steel.

The clement normally having the greatest influence is carbon.

Forms of Heat Treatment of Steel

There are different forms of heating ferrousmaterials such as steel. The methods covered in thischapter arehardening, quenching, tempering, annealingand normalizing, and case hardening. Terms such ascarburizing,cyaniding, and nitriding are also discussed.

Hardening

Heat treatment considerably transforms the grain structure of steel, and it is while passing through a critical

temperature range that steel acquires hardening power. When a piece of steel is heated slowly and uniformly

beyond a red heat, its appearance will increase in brightness until a certain temperature is reached. The color will

change slightly, becoming somewhat darker, which may be taken as an indication that a transformation is taking

place within the metal (pearlite being converted into austenite). When this change of state is complete, the steel

will continue to increase in brightness, and if cooled quickly to prevent the change from reversing, hardness willbe produced. If, instead of being rapidly quenched, the steel is allowed to cool slowly, the metal will again pass

through a change of state, and the cooling rate will be momentarily arrested.

To obtain a condition of maximum hardness, it is necessary to raise the temperature of the steel sufficiently high

to cause the change of state to fully complete itself. This temperature is known as the upper critical point. Steel

that has been heated to its upper critical point will harden completely if rapidly quenched; however, in practice,

it is necessary to exceed this temperature by approximately 28? to 56?C (50? to 100?F) to ensure thorough

heating of the inside of the piece. If the upper critical temperature is exceeded too much, an unsatisfactory coarse

grain size will be developed in the hardened steel.

Successful hardening of steel will largely depend upon the following factors:

1. Control over the rate of heating, specifically to prevent cracking of thick and irregular sections


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2. Thorough and uniform heating through sections to correct hardening temperatures

3. Control of furnace atmosphere, in the case of certain steel parts, to prevent scaling and decarburization

4. Correct heat capacity, viscosity, and temperature of quenching media, to harden adequately and to avoid

cracks

When heating steel, you should use accurate instruments to determine the temperature. At times, however, such

instruments arc not available, and in such cases, the temperature of the steel may be judged approximately by its

color. The temperatures corresponding to various colors are given in table 15-1; however, the accuracy with

which temperatures may be judged by colors depends on the experience of the worker and on the light in which

the work is being done.

QUENCHING PROCEDUREA number of liquids may be used for quenching steel. Both the media and the form of the bath depend largely on

the nature of the work to be cooled. It is important that a sufficient quantity of the media be provided to allow the

metal to be quenched without causing an appreciable change in the temperature of the bath. This is particularly

important where many articles are to be quenched in succession.

The tendency of steel to warp and crack during the quenching process is difficult to overcome because certain

parts of the article cool more rapidly than others. Whenever the transformation of temperature is not uniform,

internal strains arc set up in the metal that result in warping or cracking, depending on the severity of the strains.

Irregularly shaped parts are particularly susceptible to these conditions, although parts of an even section are

often affected in a similar manner.

Operations such as forging and machining may set up internal strains in steel parts; therefore, it is advisable to

normalize articles before attempting the hardening process. The following recommendations will greatly reduce

the warping tendency and should be carefully observed:

1. An article should never be thrown into the bath, By permitting it to lie on the bottom of the bath, it is apt to

cool faster on the top side than on the bottom side, thus causing it to warp or crack.

2. The article should be slightly agitated in the bath to destroy the coating of vapor, which might prevent it from

cooling rapidly.

Table 15-1.-Color Chart for Steel at Various Temperatures


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3. An article should be quenched in such a manner that all parts will be cooled uniformly and with the leastpossible distortion.

4. Irregularly shaped sections should be immersed in such a manner that the area with the biggest section enters

the bath first.

Quenching MediaIn certain cases water is used in the quenching of steel during the hardening process. The water bath temperature

is normally held at 18?C (65?F). For specific applications, other bath temper-atures may be used; however, cold

water may warp or crack the part, and hot water may not produce the required hardness.

A 10-percent salt brine solution is used when higher cooling rates are desired. A 10-percent salt brine solution is

made by dissolving .89 pounds of salt per gallon of water.

Oil is much slower in action than water, and the tendency of heated steel to warp or crack when quenched may

be greatly reduced by its use. Unfortunately, parts made from high-carbon steel will not develop maximum

hardness when quenched in oil unless they are quite thin in cross section. In aircraft parts, however, it is

generally used, and is recommended in all cases where it will produce the desired degree of hardness.

For many articles, a bath of water covered by a film of oil is occasionally used. When the steel is plunged

through this oil film, a thin coating will adhere to it. This action retards the cooling of the water slightly, thus

reducing the tendency to crack due to contraction.

Straightening of Parts Warped in Quenching

Warped parts must be straightened by first heating to below the tempering temperature of the article, and thenapplying pressure. This pressure should be continued until the piece is cooled. It is desirable to retemper the part

after straightening at the straightening temperature. No attempt should be made to straighten hardened steel


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without heating, regardless of the number of times it has been previously heated. Steel in its hardened condition

cannot be bent or sprung cold with any degree of safety.

Tempering

Steel that has been hardened by rapid cooling from a point slightly above its critical range is often harder than

necessary, and generally too brittle for most purposes. In addition, it is under severe internal strain. To relieve

the strains and reduce brittle-ness, the metal is usually tempered. This is accom-plished in the same types of

furnaces that are used for hardening and annealing.

As in the case of hardening, tempering temperatures may be approximately determined by color. These colors

appear only on the surface and are due to a thin

Table 15-2.-Color Chart for Various Tempering Temperatures of Carbon Steel

film of oxide, which forms on the metal after the temperature reaches 220?C (428?F). To see the tempering

colors, you must brighten the surface. When tempering by the color method, an open flame or heated iron plate

is ordinarily used as the heating medium. Although the color method is convenient, it should not be used unless

adequate facilities for determining temperatures are not obtainable. The temperatures and corresponding oxide

colors are given in table 15-2


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Retort Tempering Vacuum Furnace

Annealing and Normalizing

The most important step in annealing is to raise the temperature of the metal to the critical point, as any hardness

that may have existed will then be completely removed. Strains that may have been set up through heat treatmentwill be eliminated when the steel is heated to the critical point, and then restored to its lowest hardness by slow

cooling. In annealing, the steel must never be heated more than approximately 28? to 40?C (50? to 75?F) above

the critical point. When large articles are annealed, sufficient time must be allowed for the heat to penetrate the

metal.

Steel is usually subjected to the annealing process for the following purposes:

1. To increase its ductility by reducing hardness and brittleness.

2. To refine the crystalline structure and remove residual stresses. Steel that has been cold worked is usually

annealed to increase its ductility.

Assuming that the part to be annealed is heated to the proper temperature, the required slow cooling may be

accomplished in several ways, depending on the metal and the degree of softness required.

Normalizing, although involving a slightly different heat treatment, may be classed as a form of annealing. This

process removes all strains due to machining, forging, bending, and welding. Normalizing can only be

accomplished with a good furnace, where the temperatures and the atmosphere may be closely regulated and

held constant throughout the entire operation. A reducing atmosphere will normalize the metal with a minimum

amount of oxide scale, while an oxidizing atmosphere will leave the metal heavily coated with scale, thus

preventing proper development of hardness in any subsequent hardening operation. The articles are put in the

furnace and heated to a point above the critical temperature of the steel. After the parts have been held at this

temperature for a sufficient time to allow the heat to penetrate to the center of the section, they must be removed

from the furnace and cooled in still air. Drafts will result in uneven cooling, which will again set up strains in the

metal.


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Prolonged soaking of the metal at high temperatures must be avoided, as this practice will cause the grain

structure to enlarge. The length of time required for the soaking temperature will depend upon the mass of metal

being treated.

Case Hardening

In many instances, it is desirable to produce a hard, wear-resistant surface or "case" over a

strong, tough core. Treatment of this kind is known as "case hardening." This treatment may

be accomplished in several ways; the principal ways being carburizing, cyaniding, and

nitriding.

Carburizing

When steel is heated, the pores of the metal expand, allowing it to absorb any gases to which it is exposed. Byheating steel while it is in contact with a carbonaceous substance, carbonic gases given off by this material will

penetrate the steel to an amount proportional to the time and temperature.

The carburizing process may be applied to plain carbon steels provided they are within the low-carbon range.

Specifically, the carburizing steels are those that contain no more than 0.20 percent carbon. The lower the carbon

content in the steel, the more readily it will absorb carbon during the carburizing process. The amount of carbon

absorbed and the thickness of the case obtained increase with time; however, the carburization progresses more

slowly as the carbon content increases during the process. The length of time required to produce the desired

degree of carburization and depth of the case depend upon the composition of the metal, the kind of

carburization material used, and the temperature to which the metal is subjected. It is apparent that in

carburizing, carbon travels slowly from the outside toward the center; therefore, the proportion of carbon

absorbed must decrease from the outside to the center.

A common method of carburizing is called "pack carburizing." When carburizing is to be done by this method,

the steel parts are packed with the carburizing material in a sealed steel container to prevent the solid carburizing

compound from burning and retaining the carbon monoxide and dioxide gases. The container should be placed in

a position to allow the heat to circulate entirely around it. The furnace must be brought to the carburizing

temperature as quickly as possible, and held at this heat from 1 to 16 hours, depending upon the depth of the case

desired and the size of the work.

After carburizing, the container should be removed and allowed to cool in the air, or the parts removed from the

carburizing compound and quenched in oil or water. The air coding, although slow, reduces warpage, and is

advisable in many cases.

In another method of carburizing, called "gaseous carburizing," a carbonaceous material is introduced into thefurnace atmosphere. When the steel parts are heated in this carburizing atmosphere, carbon monoxide combines

with the iron to produce results that are practically the same as those described under the pack carburizing

process.

CyanidingSteel parts maybe surface hardened by heating while in contact with a cyanide salt, followed by quenching. Only

a thin case is obtained by this method; therefore, it is seldom used in connection with aircraft construction or

repair. However, cyaniding is a rapid and economical method of case hardening, and maybe used in some

instances for relatively unimportant parts. The work to be hardened is immersed in a bath of molten sodium or

potassium cyanide from 30 to 60 minutes. The cyanide bath should be maintained at a temperature of 760? to

899?C ( 1,400? to 1,650?F). Immediately after removal from the bath, the parts are quenched in water.

The case obtained in this manner is due principally to the formation of carbides on the surface of the steel. The

use of a closed pot is required for cyaniding, as cyanide vapors are extremely poisonous.


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NitridingThis method of case hardening is advantageous because a harder case is obtained than by carburizing. Nitriding

can only be applied to certain special steel alloys, one of the essential constituents of which is aluminum. The

process involves the soaking of the parts in the presence of anhydrous ammonia at a temperature below the

critical point of the steel. During the soaking period, the aluminum and iron combine with the nitrogen of theammonia to produce iron nitrides in the surface of the metal. Warpage of work during nitriding can be reduced

by stress-relief annealing, and by exposure to nitrogen at temperatures no higher than 538?C (1,000"F). Growth

of the work is similarly prevented, but cannot be entirely eliminated, and some parts may require special

allowance in some dimensions to take care of growth.

The temperature required for nitriding is 510?C (950?F), and the soaking period from 48 to 72 hours. An airtight

container must be used, and it should be provided with a fan to produce good circulation and even temperature

throughout. No quenching is required, and the parts may be allowed to cool in air.


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HEAT TREATMENT OF NONFERROUS METALS (ALUMINUM

ALLOYS)

Aluminum is a white, lustrous metal, light in weight and corrosion resistant in its pure state. It is ductile,

malleable, and nonmagnetic. Aluminum combined with various percentages of other metals, generally copper,

manganese, and magnesium, form the aluminum alloys that are used in aircraft construction. Aluminum alloys

are lightweight and strong, but do not possess the corrosion resistance of pure aluminum and are generally

treated to prevent deterioration. "Alclad" is an aluminum alloy with a protective coating of aluminum to make it

almost equal to the pure metal in corrosion resistance.

Several of the aluminum alloys respond readily to heat treatment. In general, this treatment consists of heating

the alloy to a known temperature, holding this temperature for a definite time, then quenching the part to roomtemperature or below. During the heating process, a greater number of the constituents of the metal are put intosolid solution. Rapid quenching retains this condition, which results in a considerable improvement in the

strength characteristics.

The heating of aluminum alloy should be done in an electric furnace or molten salt bath. The salt bath generally

used is a mixture of equal parts of potassium nitrate and sodium nitrate. Parts heated by this method must be

thoroughly washed in water after treatment. The salt bath method of heating should never be used for

complicated parts and assemblies that cannot be easily washed free of the salt.

Batch Type Aluminum Log and Billet Homogenizing Furnace

Heat Treating Procedures

There are two types of heat treatment applicable to aluminum alloys. They are known as solution and

precipitation heat treatment. Certain alloys develop their full strength from the solution treatment, while others

require both treatments for maximum strength. The NA 01-1A-9 lists the different temper designa-tions assigned

to aluminum alloys and gives an example of the alloys using these temper designations.


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Solution heat treatment

The solution treatment consists of heating the metal to the temperature required to cause the

constituents to go into a solid solution. To complete the solution, often the metal is held at a

high temperature for a sufficient time, and then quenched rapidly in cold water to retain this

condition. It is necessary that solution heat treatment of aluminum alloys be accomplished

within close limits in reference to temperature control and quenching. The temperature for

heat-treating is usually chosen as high as possible without danger of exceeding the melting

point of any element of the alloy. This is necessary to obtain the maximum improvement in

mechanical properties. If the maximum specified temperature is exceeded, eutectic melting

will occur. The consequence will be inferior physical properties, and usually a severely

blistered surface. If the temperature of the heat treatment is low, maximum strength will notbe obtained.

Solution Heat Treat Furnace and Age Oven for Automotive Castings

Precipitation hardening (aging hardening)Theprecipitation treatment consists of aging materialpreviously subjected to solution heat treatments bynatural (occurs at room temperature) or artificial aging.Artificial aging consists of heating aluminum alloy to aspecific temperature and holding for a specified lengthof time. During this hardening and strengtheningoperation, the alloying constituents in solid solutionprecipitate out. As precipitation progresses, the strengthofthe material increases until the maximum is reached.Further aging (overaging) causes the strength to decline

until a stable condition is obtained. The strengtheningof the material is due to the uniform alignment of themolecule structure of the aluminum and alloyingelement.Artificially aged alloys are usually slightlyoveraged to increase their resistance to corrosion,especially the high copper content alloys. This is donetoreduce their susceptibility to intergranular corrosioncaused by underaging.Natural aging alloys can beartificially aged;however, it increases the susceptibility of the materialto intergranular corrosion. If used, itshould be limitedto clad sheet and similar items.

Quenching

The basic purpose for quenching is to prevent the immediate re-precipitation of the soluble constituents after

heating to solid solution. To obtain optimum physical properties of aluminum alloys, rapid quenching isrequired. The recommended time interval between removal from the heat and immersion is 10 seconds or less.

Allowing the metal to cool before quenching promotes intergranular corrosion and slightly affects the hardness.


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There are three methods employed for quenching. The one used depends upon the item, alloy, and properties

desired.

Cold water quenchingSmall parts made from sheet, extrusions, tubing, and small fairings are normally quenched in cold water. Thetemperature before quenching should be 85?F or less. Sufficient cold water should be circulated within the

quenching tanks to keep the temperature rise under 20?F. This type of quench will ensure good resistance to

corrosion, and is particular y important when heat-treating 2017 and 2024 alloys.

Hot water quenchingLarge forgings and heavy sections can be quenched in hot or boiling water. This type of quench is used to

minimize distortion and cracking, which are produced by the unequal temperatures obtained during the

quenching operation. The hot water quench will also reduce residual stresses, which improves resistance to stresscorrosion cracking.

Spay quenchingWater sprays are used to quench parts formed from alclad sheets and large sections of most alloys. Principal

reasons for using this method are to minimize distortion and to alleviate quench cracking. This system is not

usually used to quench bare 2017 and 2024 due to the effect on their corrosion resistance.

Annealing

Annealing serves to remove the strain hardeningthat results from cold working and, in the case of theheat-treated alloys, to remove the effect of the heattreatment. Annealing is usually carried out in air furnaces, but salt

baths may be used if the melting point of the bath is low enough. A bath made up of equal parts by weight ofsodium nitrate and potassium nitrate is satisfactory.

Rod Annealing/Homogenizing Furnace

Annealing of work hardened materialsAnnealing of material that was initially in the soft or annealed condition but which has been strain-hardened by

cold working, such as 1100, 3003, 5052, etc., is accomplished by heating the metal to a temperature of 349 ?5?C

(660 ?10?F). It is only necessary to hold the metal at this temperature for a sufficient length of time to makecertain that the temperature in all parts of the load has been brought within the specified range. If the metal is


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Support Documents


heated appreciably above 354?C (670?F), there is a partial solution of the hardening constituents, and the alloy

will age harden while standing at room temperature unless it has been cooled very slowly. If the temperature is

not raised to 343?C (650?F), the softening may not be complete. The rate of cooling from the annealing

temperature is not important. However, a slow cool is desirable in case any part of the load may have been

heated above the recommended temperature range.

Annealing of heat-treated alloysThe heat-treatable alloys are annealed to remove the effects of strain hardening or to remove the effects of

solution heat treatment.

To remove strain hardening due to cold work, a 1-hour soak at 640? to 660?F, followed by air coding, is

generally satisfactory. This practice is also satisfactory to remove the effects of heat treatment if the maximum ofsoftness is not required.

To remove the effects of partial or full heat treatment, a 2-hour soak at 750? to 800?F, followed by a maximum

cooling rate of 50? per hour to 500?F, is required to obtain maximum softness.

To remove the effects of solution heat treatment or hardening due to cold work, the high zinc-bearing alloy 7075

should be soaked 2 hours at 775?F, air cooled to 450?, and soaked 6 hours at 450?. The stabilizing temperature at

450? is necessary to precipitate the soluble constituents from solid solution.

The annealing of solution heat-treated material should be avoided whenever possible if subsequent forming and

drawing operations are to be formed. If such operations are not severe, it is generally advantageous to repeat the

solution heat treatment and form the material in the freshly quenched condition.